I THINK I FIGURED OUT THE MYSTERY OF LUCILLE 2'S DISAPPEARANCE. Let me get the things that everyone has agreed on out of the way first: I'm almost positive that Lucille 2 suffered from vertigo from Michael trying to have sex with her again or whatever, fell down the stair car, Michael got nervous and hid the body while it was dark somehow in between fireworks, accidentally framed Buster, and then took a forget-me-now and forgot the whole thing.
Now here's the kicker: I think I know where he hid the body. Continuing off of what I said before, Michael probably hid her body in the Banana Stand which is why he inexplicably changed to a Banana Stand shirt when he gets back to the Bluth house!!!! It would also serve as a meta joke since Lucille 2 represents a lot of money for Michael and THERE'S ALWAYS MONEY IN THE BANANA STAND. Triton255 from NeoGAF also added that "Puts the "You're a crook, Captain Hook!" scene in a whole new light. With Buster as the defendant and Michael trying to convince everyone that he's guilty. Not that he necessarily would knowingly betray Buster, but still the same brother vs brother concept."
I don't think Lucille 2 is dead. I think Michael just over-panicked and hid the body in the Banana Stand only to reawaken there like GOB did in the storage unit. Plus, Michael was drunk so he probably wasn't thinking straight.
I think we cracked it.